The invention relates to a method utilizing ultrasound for determining the lateral position of a web or fabric edge in a paper machine. The invention also relates to a corresponding apparatus.
U.S. Pat. No. 5,834,877 describes an ultrasonic sensor for determining the web edge position which comprises a U-shape housing. The upper branch of the sensor housing accommodates an ultrasonic transmitter and the lower branch an ultrasonic receiver. The ultrasonic sensor is placed near the web such that the web travels through the gap between the branches. Depending on the position of the web edge, the power received by the ultrasonic receiver varies, based on which the ultrasonic sensor controller determines the edge position relative to the sensor housing. The proposed ultrasonic sensor must be positioned very near to the edge, which makes it vulnerable to damage. In addition, the web must remain accurately in place in the vertical direction, which reduces the positioning options of the sensor housing. Due to its construction and position, the sensor housing also soils easily, and its cleaning is difficult. Furthermore, the proposed ultrasonic sensor cannot be located in humid or hot conditions, which are common in a web-producing paper machine.
Due to demanding conditions, optical sensors, which are based on a photoelectric trip device, are today used in paper machines. In practice, the reflection of light directed to the fabric is detected by adjacent receivers, based on which the position of the edge can be determined. This position information is then used for example for guiding and holding the fabric in a desired position in the cross machine direction of a paper machine. The accuracy of optical sensors, however, is often insufficient. In addition, web moisture or fabric impurities affect the performance of optical sensors, which complicates the control of the apparatus and causes errors. In order to achieve sufficient accuracy, it is necessary to use several sensors, which require separate signal processing equipment for determining the position information with sufficient accuracy. This increases the purchase and operating costs of the apparatus. At the same time, using optical sensors at the so-called wet end of a paper machine is impossible. Optical sensors must also be positioned near the web or fabric, which leads to their soiling and even to damaging.